This invention relates generally to lasers and more specifically to a solid state laser in the kilowatt average power class. Lasers, with their ability to concentrate energy at a point, are potentially very useful for such industrial purposes as cutting, drilling, and welding. In order to economically justify the use of a laser to perform these industrial functions, several criteria must be met. The laser and its power supply must be competitively priced and power efficient in comparison to conventional techniques. In addition, an average power in the kilowatt range is required for most metal working operations. Continuous wave or high repetition operation is required. Very high energy pulses at low repetition rates are not useful. Repetition rates near 100 hertz are reasonable. The laser beam must be focusable to small spots, near 50 micrometers. Beam wander must be on the same order. Finally, the laser must be of a reasonable size physically, and the emitted radiation must couple well to the material being worked. That is, it should be absorbed near the surface rather than being reflected or transmitted.
Presently, Transverse Electric Atmospheric (TEA) carbon dioxide lasers are the only kilowatt average power lasers in the industrial market place. They convert electrical energy into coherent radiation with an efficiency on the order of 10 percent. However, these prior art carbon dioxide lasers exhibit three distinct disadvantages that limit their broad utility in industrial applications. First, TEA carbon dioxide (CO.sub.2) lasers require a power supply capable of delivering a very high voltage, typically 12,000 volts. Second, TEA lasers and their power supplies are physically bulky, requiring large amounts of valuable floor space. Typically, a TEA laser and its power supply occupy a space of 6 meters by 1 meter. Third, the 10.6 micron radiation produced by CO.sub.2 lasers is not absorbed well by metals, but instead is reflected. It would be advantageous to provide a physically smaller kilowatt average power laser that is capable of operation at much lower power supply voltages in the range of 440 to 2400 volts and that emits radiation having an order of magnitude shorter wavelength to guarantee much better absorption by typical industrial metals. A comparison of the radiation absorption factors of various polished metals is shown in Table 1 below taken from the Handbook of Physics.
TABLE 1 ______________________________________ % Absorption Cu Al Fe Co ______________________________________ 1.0.mu. 9.9 26.7 35.0 32.5 10.mu. 1.6 3.0 6.0 3.2 ______________________________________
High efficiency solid state lasers constructed of either glass or yttrium-aluminum garnet (YAG) doped with neodymium (Nd) are well known in the prior art. However, neither of these materials has been considered practical for the kilowatt average power application discussed above, but for different reasons.
The average power available from Nd:YAG is limited by the small size of growable cyrstals. The power from a crystal of fixed length is limited by the heat the crystal can dissipate without breaking from thermal stress. Yttrium-aluminum garnet is a crystal that is slow and expensive to grow. To date, it has been impossible to grow useful laser rods of dimensions greater than 10 millimeters in diameter and 150 millimeters in length. Few workers in this art are optimistic about improvements in YAG growth technology, though other crystalline hosts show some promise for big crystal growth. At the present time, though, the crystals are fixed at lengths of 15 centimeters. It can be shown that a YAG crystal will break from thermally-induced stress if the heat generated in the rod is greater than 115 watts per centimeter of length, regardless of the diameter of the rod. Since the heat generated is at least twice the useful power for conventionally pumped YAG lasers, the laser output is limited to about 60 watts per centimeter. Thus, a 15 centimeter rod has a maximum power output of 900 watts. This operating condition is very close to the stress fracture limit of the YAG and is not practical since the stress fracture limit of YAG varies from piece to piece. In addition, such an operating condition pushes the limit of arc lamp technology.
In the past, glass was rarely considered as a laser material in situations where high average power has been an objective. It is the material most persons skilled in the art associate with high energy per pulse and low repetition rates. The low thermal conductivity of glass makes high average power impossible with a conventional geometry. It is impossible to even approach continuous wave operation due to the low cross section for stimulated emission.
Low thermal conductivity in a laser rod means that for a given temperature difference between the center of the rod and the cooled edges, the rate of heat removal is low. The temperature difference, rather than the gradient, between the rod center and the surface is what leads to thermal stress in the rod. Thus, when thermal conductivity is low, thermal stress approaches the glass breaking point even though the rate of heat removal is low. The same equations that Koechner used to calculate the YAG heat dissipation limit can be used for glass. An LHG-5 rod will break when the heat dissipated by the rod exceeds 5 watts per centimeter of rod length, regardless of rod diameter. This fact alone would lead us to expect that a rod of length 4 meters would be required for a kilowatt laser, since again the ratio of waste heat in the rod to useful laser output exceeds two. A resonator of this dimension would be impractical.
The low cross section of Nd:glass results in low gain unless the rod is pumped very hard. It takes a powerful lamp pulse to reach the threshold of lasing in glass. Even more lamp energy is needed to reach the intensity required for saturation of the gain so as to obtain good energy efficiency. Since glass lasers of typical design are able to dissipate heat only slowly, yet require a large flashlamp energy with each pulse, only low average power, low repetition rate glass lasers are presently on the market. These devices do not meet the requirements for industrial materials processing lasers. Commercially available glass lasers generally specify repetition rates in pulses per minute, far from the approximately 100 pulses per second required of a laser successfully performing industrial operations.
Glass has two potentially major advantages in high average power laser design. First, glass can be produced in large pieces of excellent optical quality. Pieces of glass used for fusion lasers are as large as one meter square and are finished to tenth-wave flatness. The cost of the glass to make these large slabs is significantly less than the cost of crystals. The average power of glass lasers is not limited by material fabrication problems, as are YAG lasers. Second, there is a potential for efficiency from glass that is not possible in YAG. Glass at high dopings can absorb a larger fraction of the pump light than can Nd:YAG, which cannot be doped more than one atomic percent because of crystal growth problems. Glass at 7 percent doping has been claimed to have an efficiency of 6.4 percent. Lasers using YAG do not exceed 3 percent efficiency.
The problem that has prevented the use of glass as a lasant material in the prior art design of high average power lasers has been that of heat removal from the glass, which characteristically cools very slowly. It is therefore the principal object of the present invention to provide an Nd:glass high average power laser in which the gain is concentrated in a small portion of a slab of glass while the heat generated thereby is distributed throughout the entire slab of glass. This and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by moving the glass slab relative to the beam, always keeping a portion of the slab between the flash lamps but at the same time ensuring that any given portion of the slab is subjected to the output of the flash lamps for only a small fraction of the motion cycle.